1. Field of the Invention
This invention pertains in general to processes and materials for manufacturing passive electronic components. In particular, the invention relates to a novel approach for the manufacture of capacitors, varistors, resistors, inductors, and EMI filters utilizing band gap confined, nano-precision engineered materials.
2. Description of the Prior Art
In the field of electronics, passive devices such as capacitors, resistors, inductors, varistors, and EMI filters are essential components of various products and services provided by the electronics, information processing, communications, energy management, and electrical industry. For example, all modern computers, automobiles, aircrafts, communication networks and power plants utilize these components in large numbers. It is estimated that more than a hundred billion passive components are manufactured and sold every year on a world wide basis, and that the passive electronic component market in the United States exceeds $30 billion for the manufacture of products with sales in excess of $350 billions. Thus, the passive electronic component industry is fundamental to the welfare of the U.S. and world economies and any improvement in the method of manufacture or in the properties of passive components is very desirable and becoming increasingly necessary.
Existing technologies for manufacturing passive components were developed empirically at the beginning of the electronic industry over five decades ago, with significant but only incremental improvements having been achieved since then. The devices so produced have been satisfactory to meet the current needs of customers, but the recent unprecedented growth and changes in the industries utilizing passive electronic components are increasingly requiring levels of performance that conventional technologies cannot provide. For example, dramatically increasing processing speeds and operating frequencies and the unremitting need for foot print reduction of active electronic components compel progressive miniaturization for surface mount engineering and for high-frequency operation. This trend is demonstrated by the major increase in market share of miniature 0402 ceramic capacitors (0.04 inches long.times.0.02 inches wide) over the last ten years, while larger sizes, such as 0805 ceramic capacitors, have lost significant market share. However, such desirable performance characteristics cannot be met economically by current production processes and materials. Current surface mount passive components in various formulations do not offer the capacitance, resistance, inductance, EMI reduction, temperature coefficient of capacitance, temperature coefficient of resistance, temperature coefficient of inductance, break down voltage, loss factor, dissipation factor, spectrum conservation, cross talk, compatibility with conductor termination, and frequency response desired by industry. Consequently, either the performance of the intended final product is reduced in order to design around these inherent problems, or more and more volume and weight are added to the final product to account for passive electronic component requirements. With ever-growing reductions in center to center spacing of active components and with the evolution to higher frequencies in computers, higher baud rate over Internet and World Wide Web, wireless data transfer, and cellular communication, this problem represents a technology barrier.
The present invention is directed at solving this problem by employing new fabrication methods and materials based on nanosize powders such as those described in the referenced copending applications. As would be apparent to one skilled in the art, the present disclosure is presented with reference to capacitors, resistors, inductors, and varistors and improved methods for manufacturing them, but it is applicable to all types of passive electronic devices that comprise a thin layer of ceramic material as an active component. Therefore, to the extent that this invention is also applicable to capacitor arrays, resistor arrays, inductor arrays, varistor arrays, EMI filters, thermistors, piezo-devices, ceramic magnets, thermoelectric devices, ion-conducting electrolytes, batteries, and sensors, it is not intended to be limited to the manufacture of capacitors, resistors, inductors and varistors.
In response to the growing need for better performing capacitors, research and development efforts have focused on the discovery of novel materials with very high permittivity see D. Finello, "New Developments in Ultracapacitor Technology," The 4th International Seminar on Double Layer Capacitors and Similar Energy Storage Devices, Boca Raton, Fla., December (1994)!, the rationale being that high permittivity would lead to improved capacitor performance. Despite these efforts and several laboratory successes, though, these new materials have not found much market acceptance and the predominant ceramic dielectrics used in industry continue to be material formulations based on BaTiO.sub.3, SrTiO.sub.3 and TiO.sub.2. Although most new materials have a substantially increased K value, they often do not also have acceptable temperature stability (such as one matching the X7R standard); reliability data on capacitors manufactured with new materials are either not available or not sufficient to convince users of a failure rate smaller than 10 ppm, as required for acceptance in the industry; some of these materials are based on elements with too limited long-term supply for large scale use (e.g. Ruthenium); some materials are not readily suitable for large-scale automated production; and all materials are too expensive for an acceptable performance-to-cost ratio.
Therefore, the market has been reluctant to change to these new materials. Instead, techniques that have found market acceptance are those that have aimed at improving the performance of existing capacitors without changing the dielectric material. In broad terms, these techniques have involved producing ultra-thin ceramic capacitors by eliminating inhomogenities in the micron-sized dielectric layers; reducing particulate impurities in the dielectric powders used to form the dielectric layers and in the manufacturing process; and modifying raw material characteristics and manufacturing techniques to achieve electrical and mechanical strengths that are close to the theoretical values of currently used capacitor materials. The rationale for these approaches is simply based on the fact that energy density of a capacitor inversely depends on the square of the dielectric layer's thickness; that is, a reduction to one half in dielectric thickness enhances the energy density by a factor of four. Similarly, since the power density of a capacitor is a strong function of the capacitor's equivalent series resistance and inductance, both of which are a strong function of dielectric layer thickness and electrode properties, a reduction in the layer thickness can also significantly improve the power density of a capacitor. A reduction in layer thickness also reduces the device's size and weight, which is very important for applications where space and mass are premium quantities that significantly affect the overall system performance, such as in computers, portable communication, information network, and aerospace electronics. U. S. Pat. No. 5,486,277 discloses a process for manufacturing high performance capacitors by controlled sputtering of dielectric and electrode material to form a nanostructured multilayered device.
Efforts aimed at reducing particulate impurities are also significant because it is now well known that material impurities are the main cause of capacitor failure. Finally, the mechanical and electrical strength of existing capacitors is being enhanced by producing more uniform micropowder of dielectrics and by reducing adverse effect of binders. In summary, technologies that yield ultra-thin, reliable dielectric layers with less tendency to break down electrically, thermally, mechanically, or chemically during manufacturing and use, are more acceptable to and desired by the market.
Another aspect of passive-component ceramic processing addressed by this invention is the problem related to the process temperature requirements of conventional materials and manufacturing methods. As would be apparent to those skilled in the art, conventional materials and manufacturing methods require high sintering temperatures and long sintering times to form reliable, high-density ceramic layers for passive electronic components. The sintering temperature requirements often exceed 1050.degree. C. and sintering times last hours. This not only leads to increased energy costs, increased inventory costs, and increased pollution, but also limits the choice of electrode materials to expensive alloys. More specifically, the electrode composition of passive electronic components is limited to metals and alloys that can withstand temperatures in excess of 1050.degree. C., often exceeding 1,100.degree. C., such that these electrode layers can survive the sintering temperature of conventionally produced ceramic materials for electronic applications. Electrodes typically consist of a Pd/Ag alloy with some use of Pt, Cu, and Ni for special applications. This choice is an engineering trade-off between the high melting point, but high cost, of Pd and the low melting point, but low cost, of Ag. The addition of Pd (or Pt) raises the melting point of the alloy, thereby satisfying the necessity for a high melting point to meet the dielectric materials' sintering requirements. However, the low cost and a desire to utilize the excellent electrical properties of Ag (which, unfortunately, has a low melting point -960.degree. C.) have provided an incentive for using the highest possible concentration of Ag. In summary, it has been a market trend and a market need to prepare and use ceramic material formulations that offer lower sintering temperatures.
With reference to varistors, current design and production technology also present some serious limitations. Like capacitors, the varistor production process begins by mixing micron-sized powders corresponding to a given ceramic material formulation. The dominant ceramic composition for varistors is based on ZnO and SrTiO.sub.3, supplemented with additives such as Bi.sub.2 O.sub.3, CoO, MnO, Cr.sub.2 O.sub.3, and Sb.sub.2 O.sub.3. The powders are milled in deionized water. The mix is then typically blended with PVA and spray dried. The resulting granulated powder is pressed to disc (or any other desired shape) with a diameter ranging between 0.4 and 10 cm and a thickness ranging between 0.1 and 4 cm. The pressed pellet is sintered at about 1,200.degree. C. in air and then metallized to make electrodes on the disc faces. The electrode surface is then soldered with leads for connections. Alternatively, multilayer varistor devices are prepared, like capacitors, by stacking layers of ceramics separated by electrode layers. Finally, this assembly is environmentally and mechanically protected by encapsulation using a conforming coating or other packaging techniques.
Research and development efforts in the field of varistors have focussed on the discovery of novel additives and materials, on heat treatment methods to control grain growth, on disc thickness reduction, on layer thickness reduction, on sintering temperature reduction, and on formation techniques for multilayer varistors, for low voltage and high frequency applications. Despite these efforts, current varistor technology continues to face several limitations. Existing varistors are bulky, with diameters between 0.4 cm and 10 cm, and thicknesses of 0.1 cm to 4 cm. This size limitation is significant in applications, such as for computers, portable communication, information network, and aerospace electronics, where weight and space are important considerations. Furthermore, the trend towards miniaturization of solid-state devices imposes further size reduction requirements on existing varistor technology. The capacitance range of current varistors is between 50 pF and 5000 pF, which precludes operation in high frequency applications. In addition, ZnO varistors typically feature a 3 V/grain voltage drop; therefore, the threshold voltage for ZnO varistors is often over 10 volts. This makes it difficult to integrate miniature varistors with electronic components and devices working at voltages below 10 V. The use of SrTiO.sub.3 may provide a material breakthrough because it features a voltage drop of 1.3 V/grain. However, the size and frequency limitations apply also to SrTiO.sub.3 varistors.
The ceramic powders used for varistor manufacturing have grain sizes from 1 .mu.m to as large as 10 to 25 .mu.m. As a result, the sintering step during varistor manufacturing necessarily requires temperatures in the 1100 to 1300.degree. C. range, which leads to significant grain growth. Given the inherent limitations associated with the size of precursor powders, the only strategy that is practically available with prior-art technology to achieve a high reference breakdown limit voltage is to increase the varistor thickness (i.e. make the varistor bulky).
Attempts to miniaturize varistors for low voltage applications are also limited by the powder size. Like capacitors, in varistors the ceramic layer thickness is a function of the starting ceramic powder size. Thus, while electronic devices are increasingly reducing in foot print size and line to line spacing to sub-micron levels, the varistor component cannot be reduced beyond a size of a few microns. Finally, the present invention also addresses the unsatisfactory performance of conventional varistors in high frequency applications. This limitation can be traced back to large grain sizes and to the increase of losses with increases in grain size and frequency.
With reference to resistors, existing precursors (e.g. resistor inks) that are used to prepare film resistors are based on micron-sized powders. More specifically, the blend often consists of glass frit, electrically conducting powder, additives and an organic screening agent. The existing resistor inks are increasingly proving to be a design limitation where very low sheet resistivities and high frequencies are required. Furthermore, with ongoing and anticipated device miniaturization, higher density packing of circuit features and consequent reduction in footprint, it is expected that existing resistor fabrication technology will be limited by micron sized powders being used as resistor precursors.
Users of passive electronic components desire resistors with high sheet resistance, high frequency response, high power dissipation, and high voltage operability, which is difficult to achieve with conventional materials in miniaturized resistors. The teachings of this invention are also applicable to satisfy this need and to improve other performance limitations in resistor technology, such as low temperature coefficient of resistivity (TCR), high short-time overLoad voltage (STOLV), low current noise, minimum permanent changes in resistance with elevated temperatures, humidities and stress, compatibility with conductor terminations, and low processing sensitivity.
With respect to inductors, the ceramic materials of choice are often ferrites and rare earth borides. Once again, like capacitors, varistors and resistors, conventional inductor manufacturing is based upon the use of micron-sized powders of magnetic ceramics. The micron-sized magnetic ceramic powders are usually prepared by milling, attrition, precipitation, or a combination of these techniques. The powders are formulated with additives and processing aids, processed into desired shapes such as layers, discs, chips, toroids, and tubes. The material is then provided with an electrode layer to form the single layer inductive component. For multilayers, alternating layers of magnetic ceramic and electrodes are provided. The device is then singulated, if needed, and sintered at temperatures in excess of 1050.degree. C. As a final step, the device is often environmentally shielded with a layer of protective material, usually a polymer.
This invention addresses the limitation in existing inductors to operation at low frequencies and low energy densities, and the limitations to miniaturization and reduction of inductors' foot print. Like capacitors, varistors and resistors, conventional inductors prepared from micron sized powders are limited in final component foot print to sizes that are multiples of the starting powder size (that is, multiples of microns). In the case of multilayered components, final component foot prints are limited to sizes that are even larger (often multiples of millimeters). In addition, this invention addresses the need for materials with higher resistivity and higher initial permeability.
Regarding EMI filters, these passive components are extensively used to filter noise, reduce cross talk, preserve electromagnetic spectrum in electronic, electrical, information, computing, and magnetic devices. As would be apparent to the skilled in the art, an EMI filter is a combination of resistive, inductive, and capacitive elements. Therefore, all problems and limitations in these components are also applicable to EMI filters. Consequently, existing EMI filter components exhibit poor performance at high frequencies, are bulky, expensive to manufacture, and are difficult to miniaturize.
Thus, recent competitive trends in passive electronic component manufacturing have been towards making the thickness of each layer in the component thinner, reducing the sintering temperatures so as to increase the use of Ag as electrode material, enhancing the material properties, reducing component costs, increasing component reliability, and improving the productivity of each processing step. A technological breakthrough that enabled greater miniaturization and provided improved high-frequency performance characteristics would be very desirable. The present invention provides a novel approach that greatly improves these aspects of passive electronic component performance and manufacture.